A. Field of the Invention
The present disclosure relates, generally, to transparent conducting materials (“TCMs”) and, more particularly, to hybrid TCMs including a polycrystalline film that is “percolation doped” with conductive nanostructures.
B. Description of the Related Art
Transparent conducting electrodes (TCEs) require high transparency and low sheet resistance for applications in photovoltaics, photodetectors, flat panel displays, touch screen devices and imagers. Indium tin oxide (ITO), or other transparent conductive oxides, have typically been used, and provide a baseline sheet resistance (RS) vs. transparency (T) relationship. However, ITO is relatively expensive (due to limited abundance of indium), brittle, unstable, inflexible. It increases in brittleness with aging and is chemically unstable under acid/base conditions. ITO transparency drops rapidly for wavelengths above 1000 nm, so it has poor transmittance in the near infrared. Furthermore, metallic-ion diffusion from ITO into thin barrier layers may result in parasitic leakage. These and other problems make ITO-based technologies non-ideal for applications such as thin film photovoltaics (“PVs”), flexible electronics, touch-screen displays, light emitting diodes, and the like.
A suitable replacement for ITO is desired therefore. However, since resistivity and transmittance are often fundamentally constrained by the intrinsic properties of a material, developing TCMs with both low sheet resistance (e.g., Rs<10Ω/□) and high transmittance (e.g., T>90%) has been a persistent challenge. Various alternative TCMs to ITO have been explored, including, by way of example, networks of carbon nanotubes (“CNTs”) and networks of metal nanowires (“NWs”). In networks of silver nanowires (AgNWs nanonet) and single-wall carbon nanotubes (SWCNTs nanonet), for NW or CNT densities corresponding to 85-95% transparency (T), conduction is typically dominated by percolation through junctions with relative large tube-tube/NW-NW contact resistance (RNW-NW), resulting in a rapid increase in baseline sheet resistance (RS) (kΩ/□-GΩ/□, depending on the NW/NT) as T increases. Networks of only metallic nanowires exhibit sheet resistance of the order of kilo-ohm/□ and more. Approaches involving welding of the nanowires, thermal annealing under pressure, or electroplating decrease RS by improving RNW-NW, but it has been challenging to reduce overall RS below≈30Ω/□, especially for broadband T at 90%. Moreover, micrometer-sized holes within the network add series resistance to devices that rely on vertical current transport such as LEDs and solar cells. Composite transparent conducting electrodes (TCEs) employing silver NWs with another conducting polymer such as PEDOT:PSS and a combination of TiO2 nanoparticles with PEDOT:PSS have recently been demonstrated with sheet resistances of 12Ω/□ at average T of 86% over wavelengths 350-800 nm and 15Ω/□ at T550 nm of 83% respectively. The conducting polymer and TiO2 nanoparticle primarily reduce the tube-tube contact resistance.
Other alternative TCMs to ITO have been explored, including chemical vapor deposited (“CVD”) polycrystalline graphene (“poly-graphene” or “PG”) films, including single layer graphene (SLG) and few-layer graphene (FLG). “Single-crystal” graphene, such as that obtained by exfoliation from highly ordered pyrolytic graphite (HOPG) crystals, exhibits several interesting physical phenomena, including an RS lower than ITO, at a given optical transparency. Single-layer graphene (SLG) or few-layer graphene provide sufficiently high transparency (≈97% per layer) to be a potential replacement for ITO. However, the exfoliated approach yields samples that are too small for practical applications, and large-area synthesis approaches, including chemical vapor deposition (CVD), typically involving growth on copper foil and subsequent transfer to an arbitrary substrate, produces grain sizes typically ranging from a few micrometers to a few tens of micrometers, depending on the specific growth conditions. The resulting films have relatively high sheet resistance due to small grain sizes and high-resistance grain boundaries (HGBs).
While these potential ITO replacements each resolve several practical issues associated with ITO, their respective Rs-T curves are not significantly different from that of ITO (as shown in FIG. 9). To achieve technologically relevant sheet resistance values (e.g., Rs<20Ω/□), the density of a network of CNTs or NWs must significantly exceed the percolation threshold. These high densities of CNTs or NWs, however, reduce the transmittance of such TCMs considerably. Moreover, even with low Rs, vertical current collection in PV cells is compromised by current crowding at the small-area interface between a network of CNTs or NWs and the bulk emitter layer. Meanwhile, experimental data suggests that there is a fundamental limitation to the sheet resistance and transmittance of pure poly-graphene films, making it difficult for poly-graphene to compete successfully with ITO.
It is therefore desired to produce an alternative to ITO that simultaneously exhibits high transparency and a technologically relevant sheet resistance value (e.g., Rs<20Ω/□).